Frigatebirds are a family of seabirds which have a scope for strong sexual selection. This due to the striking ways that males attract females and that through observation in a study population, only 55% of males successfully obtained a mate. The species used for the study was the Magnificent frigatebird, Fregata magnificens.

When females are looking for a mate, they fly around male display sites and look for that bright red pouch being inflated by the males. This is the gular pouch that males present along with a drumming sound. When females are inspecting, males turn their body to makes their pouch look as big as possible and produce loud and deep drumming sounds. As it may be difficult for females to accurately assess the males due to height or the way the male is positioned, it was thought that there was a relationship between pouch size and the frequency of the drumming sound and that the pouch acts as some kind of resonance chamber.

69 males were assessed during their courtship, their pouch sizes noted, and their drumming sounds recorded for analysis. It was found that there was a negative correlation between the two variables. As the size of the pouch increased, the frequency of the drumming sound decreased thus providing evidence to support the hypothesis that the pouch acts as a resonance chamber. This correlation suggests that, as the females might not be able to accurately see the pouch size, the frequency of the drumming sound helps them get an idea and therefore helps them make their decision.

Reference

Madsen V, Balsby T. J, Dabelsteen T & Osorno J. L. (2004). Bimodal signaling of a sexually selected trait: gular pouch drumming in the magnificent frigatebird. The Condor, 106(1), 156-160.